1. Field of the Invention
The present invention relates to a transmission equipped with a sliding mechanism that enables a sprocket, around which a chain is wrapped, to move in an axial direction, relative to a shaft on which the sprocket is mounted. The transmission is provided, for example, for bicycles. More particularly, the invention relates to a transmission mechanism of the type having a protection mechanism against foreign objects.
2. Background Art
A transmission in which a gear shift is performed by shifting a chain among a plurality of sprockets includes a transmission for bicycles that is disclosed in Japanese Laid-Open Patent document No. 2004-155280. This transmission includes: a transmission mechanism constituted of a drive sprocket which is coupled to a crankshaft in a driven manner, a plurality of transmission sprockets arranged in the axial direction, and a chain which is wrapped around both one of the plurality of transmission sprockets and the drive sprocket; and a derailer for shifting the chain among the plurality of transmission sprockets. The transmission mechanism and the derailer are housed in a transmission case. When the derailer shifts the chain from one transmission sprocket to another, the chain moves in the axial direction and hence the drive sprocket also moves in the axial direction. Thus, a sliding mechanism is provided between the crankshaft and the drive sprocket, for causing the drive sprocket to rotate integrally with the crankshaft and for enabling the drive sprocket to move along the crankshaft in the axial direction. The sliding mechanism is constituted of a ball spline mechanism which includes an inner cylinder that can rotate integrally with the crankshaft, an outer cylinder with the integrated drive sprocket, and a plurality of balls that are arranged between the inner and outer cylinders. The plurality of balls are housed between a pair of housing grooves respectively formed on the outer side surface of the inner cylinder and the inner side surface of the outer cylinder along the axial direction. The balls come in contact with guide surfaces, which are wall surfaces of the housing grooves of the inner cylinder, and roll. Thus, the balls are guided to the guide surfaces and thereby the drive sprocket moves in the axial direction integrally with the outer cylinder.
In a transmission equipped with sprockets and a chain, the chain is engaged with the sprockets and is brought into contact with a derailer, whereby abrasion powder are generated. The abrasion powder sometimes enters tramsmission grooves. If the abrasion powder attaches to the guide surfaces (wall surfaces) of the grooves of a sliding mechanism, the smooth rolling of balls is inhibited. Thus, the drive sprocket cannot move in the axial direction smoothly. The attachment of abrasion powder to the guide surface of the housing grooves is difficult to identify visually when a transmission mechanism is housed in a case. For this reason, when maintenance is performed, the case must be opened frequently to check the degree of attachment of abrasion powder.